Radiolabelling of this drug candidate for the treatment of influenza will enable us to directly demonstrate that it binds to the H1, H2, and H3 sites on the target protein(s), and will enable us to determine dissociation constants. In addition, this will enable us to study the mechanism of certain antagonistic compounds, which we believe can bind to the target protein but do not inhibit it. The other two compounds are structurally similar derivatives which contain a photoactivable azide linkage. These two radiolabelled compounds will be used in an attempt to identify the binding pocket within the target protein for this class of inhibitors.